The present invention relates to a polarization-mode dispersion detecting method, and a dispersion compensation controlling apparatus and a dispersion compensation controlling method used when polarization-mode dispersion or chromatic dispersion of a transmission optical signal which becomes a factor of limitation on a transmission distance of a high-speed optical signal in a very high-speed optical communication system adopting, for example, optical time division multiplexing.
In a trunk-line optical communication system, a system with a transmission rate 10 Gb/s (gigabit/second) is in stage of practical application. On the other hand, there is a demand for a larger capacity of the optical communication system with a rapid increase of an information quantity. Considered as candidates for employable system are time division multiplexing (including optical time division multiplexing) and wavelength division multiplexing. Particularly, in time division multiplexing, a lot of researches on a very high-speed optical communication system with a transmission rate 40 Gb/s (hereinafter referred to as a 40 Gb/s optical communication system) are conducted inside and outside the country.
However, the 40 Gb/s optical communication system has a problem that a transmission distance of an optical signal is limited since a transmission waveform is deteriorated by effects of polarization-mode dispersion and chromatic dispersion. Namely, in this system transmission line, a chromatic dispersion value and a polarization-mode dispersion value are factors of limitations of a transmission rate and a transmission distance. Hereinafter, results of simulation and results of experiment on chromatic dispersion will be described with reference to FIGS. 66 through 72, and polarization-mode dispersion will be described with reference to FIGS. 73 through 75.
Although a term xe2x80x9cdispersionxe2x80x9d is generally used to mean xe2x80x9cchromatic dispersionxe2x80x9d, when merely the term xe2x80x9cdispersionxe2x80x9d is used hereinafter, it means both xe2x80x9cpolarization-mode dispersionxe2x80x9d and xe2x80x9cchromatic dispersionxe2x80x9d unless specifically mentioned.
First, chromatic dispersion will be schematically described. Since a chromatic dispersion tolerance (tolerance means an allowance) is inversely proportional to the square of a bit rate, a chromatic dispersion tolerance of 10 Gb/s is 800 ps/nm, while a chromatic dispersion tolerance of 40 Gb/s is about 50 ps/nm that is one sixteenth of 800 ps/nm, which is severer.
FIG. 66 shows a structure of an experimental system to evaluate dispersion compensation tolerance after 50 km transmission over a 1.3 xcexcm zero-dispersion fiber (SMF: Single Mode Fiber) in 40 Gb/s optical time division multiplexing (OTDM: Optical Time Division Multiplexing). Here are used a chromatic dispersion value=18.6 ps/nm/km, and a total dispersion value=930 ps/nm. A 40 Gb/s optical transmitter 121a shown in FIG. 66 is a signal light source. A signal light intensity-modulated in an intensity modulator 121b is inputted to a receiving side (hereinafter referred to as a receiving terminal, occasionally) over a DCF (Dispersion Compensating Fibers) 124 via the SMF 123. On the receiving side, a preamplifier 122a and a 40 Gb/s optical receiver 122b perform a demodulating process.
FIG. 67 shows a result of an evaluation experiment in this experimental system, wherein a transverse axis represents total dispersion quantity (unit: ps/nm) while a vertical axis represents power penalty (unit: dB). If here is required a power penalty 1 dB or less as an evaluation reference of the transmission line, a dispersion compensation tolerance (dispersion width) is 30 ps/nm, this value corresponding to 2 km or less in transmission using SMF. Namely, when a repeater spacing, that is, a distance between stations, is not constant as in a ground system, it is necessary to optimize a dispersion compensation quantity (high-accuracy dispersion compensation of about 100%) of each repeater section.
Additionally, a chromatic dispersion value of an optical fiber transmission line changes with time with a change of laying environment such as temperature, pressure and the like. For example, in the case of a change in temperature from xe2x88x9250 to 100xc2x0 C., a quantity of the change in dispersion of SMF 50 km is estimated to be 16 ps/nm as shown by the following formula:                               [                      dispersion            ⁢                          xe2x80x83                        ⁢            change            ⁢                          xe2x80x83                        ⁢            quantity                    ]                =                  xe2x80x83                ⁢                              [                          temperature              ⁢                              xe2x80x83                            ⁢              dependency              ⁢                              xe2x80x83                            ⁢              of              ⁢                              xe2x80x83                            ⁢              zero              ⁢                              xe2x80x83                            ⁢              dispersion              ⁢                              xe2x80x83                            ⁢              wavelength                        ]                    xc3x97                                                  xe2x80x83                ⁢                              [                          temperature              ⁢                              xe2x80x83                            ⁢              change                        ]                    xc3x97                      [                          dispersion              ⁢                              xe2x80x83                            ⁢              slope                        ]                    xc3x97                                                  xe2x80x83                ⁢                  [                      transmission            ⁢                          xe2x80x83                        ⁢            distance                    ]                                        =                  xe2x80x83                ⁢                  0.03          ⁢                      xe2x80x83                    ⁢                      (                                          nm                /                xc2x0                            ⁢                              xe2x80x83                            ⁢                              C                .                                      )                    xc3x97          150          ⁢                      xe2x80x83                    ⁢                      (                          xc2x0              ⁢                              xe2x80x83                            ⁢                              C                .                                      )                    xc3x97          0.07          ⁢                      xe2x80x83                    ⁢                      (                                          ps                /                                  nm                  2                                            /              km                        )                    xc3x97          50          ⁢                      xe2x80x83                    ⁢                      (            km            )                                                  =                  xe2x80x83                ⁢                  16          ⁢                      xe2x80x83                    ⁢                      ps            /            nm                              
This value is more than a half of the dispersion tolerance 30 ps/nm, which has to be considered in full in system designing. The reason is that when a temperature becomes 100xc2x0 C. during system operation, he value does not meet the reference of penalty 1 dB in the worst case, even if the dispersion compensation quantity is optimized at xe2x88x9250xc2x0 C. when the operation of the system is started. Depending on characteristics or a structure of the dispersion compensator, it is impossible to continuously set a dispersion compensation quantity, so that there is a case where the dispersion compensation quantity can be set to only a value slightly deviated from an optimum value when the operation of the system is started. In this case, the value might not meet the reference of penalty 1 dB even with a change in temperature below 150xc2x0 C.
In the above consideration, in order to realize a very high-speed optical communication system above 40 Gb/s, it is necessary to first optimize dispersion equalization (dispersion compensation quantity) in each repeater section when the system operation is started, and to secondary configure xe2x80x9can automatic dispersion equalization (compensation) systemxe2x80x9d optimizing dispersion equalization (dispersion compensation value) correspondingly to a change with time of a transmission line dispersion value even during the system operation. Meanwhile, this automatic dispersion equalization system is required not only in the SMF transmission system but also in the case where a 1.55 xcexcm wavelength dispersion shifted fiber (DSF: Dispersion Shifted Fiber) having a small chromatic dispersion value is used. Elemental techniques for realizing the automatic dispersion equalization system are summarized into three points, (a) through (c) below:
(a) realization of a variable dispersion equalizer (compensator);
(b) method of monitoring a chromatic dispersion value (or a total dispersion quantity after dispersion equalization [compensation]) of a transmission line; and
(c) method of controlling feedback optimization of a variable dispersion equalizer (compensator).
As a method of measuring a chromatic dispersion value of an optical fiber, there has been used a pulse method or a phase method in which light having plural different wavelengths is inputted to an optical fiber, and a group delay difference or a phase difference in the output light is measured. However, in order to always measure dispersion during the system operation using these methods, a set of chormatic dispersion measuring devices are required in each repeater section. Further, in order to measure a dispersion quantity without interrupting transmission of data signal light, it is necessary to wavelength-multiplex measuring light having a wavelength different from that of the data signal light.
Assembling the pulse method or the phase method in an optical transmission apparatus is not realistic from the points of view of size and economy. Further, when a wavelength different from that of the main signal light, there is a possibility of lacking accuracy since it is necessary to perform a process to assume a dispersion value at a wavelength of the signal light from a measured value at a wavelength of the measuring light. For this, a method being able to directly monitor a wavelength dispersion value from the main signal light is desirable.
As this wavelength dispersion monitoring method, there has been already proposed in the Conferrence and the like a method using a 40 GHz component intensity in a baseband spectrum of a 40 Gb/s OTDM signal and an NRZ (Non-Return-to-Zero) signal.
FIG. 68 shows a relationship (simulation results) between 40 GHz component intensity and eye-opening with respect to dispersion quantity of a 40 Gb/s OTDM signal. Between two curves shown in FIG. 68, one having a pair of peaks represents 40 GHz component intensity, while the other one having a single peak represents eye-opening, wherein the minimum point between the pair of peaks of the 40 GHz component intensity is zero dispersion point, at which the eye-opening is the maximum.
FIG. 69 shows a structure of an experimental system at the time of DSF 100 km transmission. Signal light is sent from a transmitting side (hereinafter referred to as a transmitting terminal, occasionally) 131 shown in FIG. 69, and a temperature of a fiber that is a transmission line can be changed in a thermostat 133. On a receiving side 132, a 40 GHz component intensity is measured.
FIG. 70 shows results of the experiment in the experimental system, wherein a transverse axis represents signal light wavelength, while a vertical axis represents monitor voltage at a 40 GHz component intensity. The signal light wavelength that is the transverse axis is swept in a range from 1535 to 1565 nm [nanometer: (nano represents the minus ninth power of 10)], while the monitor voltage represents results at three kinds of temperatures. In each of these three kinds of waveforms, the minimum point between a pair of peaks of the waveform shows zero dispersion wavelength, like the simulation result shown in FIG. 68. Following a change in temperature (xe2x88x9235 to +65xc2x0 C.) of DSF 100 km, it is known that the zero dispersion wavelength is changed (0.027 nm/xc2x0 C.).
FIG. 71(a) shows a relationship (simulation results) between 40 GHz component intensity and eye-opening with respect to a dispersion quantity of a 40 Gb/s NRZ signal (xcex1=xe2x88x920.7). In FIG. 71(a), one having a plurality of peaks represents 40 GHz component intensity, while the other one having a single peak represents eye-opening, as well. In the case of xcex1 less than 0, the 40 Gb/s component intensity has the maximum peak in the vicinity of +30 ps/nm, and the monitor value shows zero that is the minimum value in zero dispersion at the foot on the negative dispersion""s side.
FIG. 71(b) shows results of an experiment at the time of DSF 100 km transmission when a temperature is changed from xe2x88x9235 to +65xc2x0 C. As well as the simulation results [refer to FIG. 71(a)], the minimum value at the foot on the long wavelength""s side of the maximum peak [refer to a point denoted by 134 in FIG. 71(b)] shows zero dispersion wavelength, and the zero dispersion wavelength is changed at 0.026 nm/xc2x0C., which coincides with the results in FIG. 70. FIG. 71(a) shows simulation results in the case of a 40 Gb/s NRZ signal (xcex1=+0.7). FIG. 72(b) shows simulation results in the case of a 40 Gb/s RZ (Return-to-Zero) signal (xcex1=0, Duty=50%). In such the automatic dispersion compensation system, it is necessary to feedback-control an operation point of a variable dispersion (equalization) compensator such that the eye-opening becomes the maximum using the above chromatic dispersion monitor.
Next, polarization-mode dispersion (PMD: Polarization-Mode Dispersion) that is the second factor having an effect on a transmission distance in the 40 Gb/s system will be schematically described. Polarization-mode dispersion (PMD) is caused by that propagation delay times of polarization components (light in two modes such as TE mode and TM mode, for example) of a light signal are different, which might generate in any optical fiber. Generally, the larger a transmission rate of an optical signal or the longer a transmission distance of an optical signal, the larger is an effect of polarization-mode dispersion, which cannot be ignored. It is said that some optical fibers configuring old optical transmission lines laid mainly in countries other than Japan have a large PMD value above 1 ps/km1/2 [picosecond/km1/2 (pico represents minus twelfth power of 10) per unit length. In the case of a short distance transmission (for example, 50 km transmission) using such optical fibers, an optical delay difference (xcex94xcfx84) is 7 ps or larger per one time slot 25 ps of 40 Gb/s, where an effect of polarization-mode dispersion cannot be ignored. Incidentally, this value is determined according to a type of optical fiber, which does not depend on a transmission rate of an optical signal. Further, since it is practically necessary to provide devices generating polarization-mode dispersion such as an optical amplifier, a wavelength dispersion compensator and the like in an optical communication system, a transmission distance of an optical signal is further limited.
Accordingly, in order to increase a transmission rate of an optical signal while still using an optical transmission line having been already laid or perform long-distance in-line repeater transmission while still using an optical transmission line having been already laid, a technique of compensating polarization-mode dispersion generated in a transmit optical signal is demanded.
As methods of compensating polarization-mode dispersion, there are compensating methods described in publications shown below, for example. Incidentally, it is difficult to thoroughly compensate transmit waveform deterioration since mode coupling due to fluctuation of birefringence in a longitudinal direction of an optical fiber is complicatedly generated even with an optical fiber configuring an actual optical transmission line, moreover, the mode coupling is changed with time due to temperature change and the like. In order to relieve transmission waveform deterioration, methods described in publications {circle around (1)} through {circle around (3)} shown below are effective.
{circle around (1)} Method of providing a polarization controller (PC: Polarization Controller) at a transmitting terminal of an optical signal, feeding back transmission characteristic from the receiving terminal so as to control a splitting ratio xcex3 of an optical intensity to two polarization modes to be 0 or 1 (J. H. Winters et al., xe2x80x9cOptical equalization of polarization dispersionxe2x80x9d, SPIE Vol.1787 Multigigabit Fiber Communications, 1992, pp.346-357).
{circle around (2)} Method of providing a polarization controller and a polarization maintaining fiber (PMF: Polarization Maintaining Fiber) at a receiving terminal of an optical signal, and controlling the polarization controller to give a delay difference (fixed value) between two polarization modes of an inverse code to an optical transmission line (T. Takahashi et al., xe2x80x9cAutomatic compensation technique for timewise fluctuating polarization-mode dispersion in in-line amplifier systemsxe2x80x9d, Electro.Lett., vol.30, No.4, 1994, pp.348-349); and
{circle around (3)} Method of providing a polarization controller, a polarization beam splitter (PBS: Polarization Beam Splitter), photo receivers receiving two optical signal components split by the polarization beam splitter, and a variable delay element giving a delay difference between two electric signals obtained by the photo receivers to control the polarization controller and the variable delay element (T. Ono et al., xe2x80x9cPolarization Control Method for Suppressing Polarization-mode Dispersion Influence in Optical Transmission Systemsxe2x80x9d, J. of Lightwave Tecnol., vol.12, no.5, 1994, pp.891-898).
In any of these methods {circle around (1)} through {circle around (3)}, it is necessary to detect a state of polarization-mode dispersion at a receiving terminal of an optical signal to perform a feed-back control. However, there is required not a complicated method using a result of detection of a code error rate or the like but a technique of easily detecting a state of polarization-mode dispersion. Such an optical communication systems will be required in future that a bit rate, a transmission distance, a signal modulation format and the like can be freely changed. For this, even in a technique of compensating polarization-mode dispersion, it is required to comply with fluctuations of a state of polarization-mode dispersion generated in a transmission line.
FIG. 73 shows an experimental system for studying transmission waveform deterioration of a 40 Gb/s signal by PMD. An optical intensity splitting ratio (or an optical power ratio) xcex3 of each polarization component of signal light sent out from a transmitting side 133 shown in FIG. 73 is changed in a polarization controller 134, the signal light is added PMD generated in a transmission line in a PMD emulator (PMD emulator) 135 and demodulated in a receiving terminal 136. The PMD emulator 135 simulates PMD generated in the transmission line, wherein a commercially available PMD emulator is used. Principles upon which the PMD emulator 135 operates are as follows. Namely, the signal light is split into two polarization components by the polarization beam splitter (PBS) 135a shown in FIG. 73, one of which is given an optical delay difference xcex94xcfx84 (ps) in an optical delay device 135b, the other of which is given a loss in an optical attenuator 135 such that optical losses in the both optical paths are equal. Further, they are multiplexed while they are still in an orthogonal state by a polarization beam splitter (PBS) 135d. The output signal is amplified by an optical preamplifier 136a in the receiving terminal 136, and demodulated in an optical DEMUX (Demultiplex) 136b. 
FIG. 74 shows results of an experiment of evaluation of power penalty to optical delay difference xcex94xcfx84 of a 40 Gb/s OTDM signal and a NRZ signal. The transversal axis represents optical delay difference xcex94xcfx84, while a vertical axis represents power penalty. Incidentally, xcex3 is set to 0.5 in the polarization controller 134 (refer to FIG. 73) such that transmission waveform deterioration is the maximum. A curved line denoted by (a) in FIG. 74 represents transmission waveform deterioration of the OTDM signal. When a reference value of receiver sensitivity degradation (power penalty [vertical axis]) is below 1 dB, a PMD allowable value (PMD tolerance) is 9 ps. A curved line denoted by (b) in FIG. 74 represents transmission waveform deterioration of the 40 Gb/s NRZ signal. When a reference value of receiver sensitivity degradation at this time is below 1 dB, the PMD allowable value (PMD tolerance) is 11 ps.
In consideration of a value of the receiver sensitivity degradation, some relatively old fibers having been already laid have a large PMD value above 1.0 ps/km1/2 per unit length. In such case, a value of the receiver sensitivity degradation is above 10 ps even in a relatively short distance transmission of 100 km or less. Further, since polarization-mode dispersion is generated even in an optical amplifier, a chromatic dispersion compensator and the like other than a transmission line fiber in an actual optical transmission system, a transmission distance is further limited. In order to increase a transmission distance in a fiber transmission line having been already laid, or in order to perform long-distance in-line repeater transmission, xe2x80x9cPMD compensating techniquexe2x80x9d is required. However, this compensating technique has three problems (d) through (f) below.
(d) realization of a PMD compensating device;
(e) method of detecting a PMD state (optical delay difference xcex94xcfx84 and optical intensity splitting ratio xcex3); and
(f) method of controlling feedback-optimization of a PMD compensating device.
Although a PMD measuring device has been commercially available, introducing such PMD measuring device as a part of an optical transmission system is not realistic in the view of size and economy. A method being able to directly monitor a PMD value is desirable. As such method, there is a method using a frequency component intensity in a baseband spectrum of a received signal, which is theoretically determined as below.
Assuming that F(t) is a change of an optical intensity with time when PMD is not given, a change of an optical intensity with time when PMD (optical delay difference xcex94xcfx84 and optical density splitting ratio xcex3) is given by the following formula:
xcex3F(txe2x88x92xcex94xcfx84)+(1xe2x88x92xcex3)F(t)
An electric field intensity of an electric signal having been received is proportional to its value, and the square of the value is detected as a change of the intensity with time by the intensity detector. Baseband spectrum P(f) is expressed as its Fourier transform by the following formula (11):                                                                         P                ⁡                                  (                  f                  )                                            =                              xe2x80x83                            ⁢                                                "LeftBracketingBar"                                      ∫                                                                                            {                                                                                    γ                              ⁢                                                              xe2x80x83                                                            ⁢                                                              F                                ⁡                                                                  (                                                                      t                                    -                                    Δτ                                                                    )                                                                                                                      +                                                                                          (                                                                  1                                  -                                  γ                                                                )                                                            ⁢                                                              F                                ⁡                                                                  (                                  t                                  )                                                                                                                                              }                                                ·                                                  exp                          ⁡                                                      (                                                          ⅈω                              ⁢                                                              xe2x80x83                                                            ⁢                              t                                                        )                                                                                              ⁢                                              ⅆ                        t                                                                              "RightBracketingBar"                                2                                                                                        =                              xe2x80x83                            ⁢                                                "LeftBracketingBar"                                                            γ                      ⁢                                              ∫                                                                              F                            ⁡                                                          (                                                              t                                -                                Δτ                                                            )                                                                                ⁢                                                      exp                            ⁡                                                          (                                                              ⅈω                                ⁢                                                                  xe2x80x83                                                                ⁢                                t                                                            )                                                                                ⁢                                                      ⅆ                                                          xe2x80x83                                                        ⁢                            t                                                                                                                +                                                                  (                                                  1                          -                          γ                                                )                                            ⁢                                              ∫                                                                              F                            ⁡                                                          (                              t                              )                                                                                ⁢                                                      exp                            ⁡                                                          (                                                              ⅈω                                ⁢                                                                  xe2x80x83                                                                ⁢                                t                                                            )                                                                                ⁢                                                      ⅆ                            t                                                                                                                                "RightBracketingBar"                                2                                                                                        =                              xe2x80x83                            ⁢                                                "LeftBracketingBar"                                                                                    γexp                        ⁡                                                  (                          ⅈωΔτ                          )                                                                    ⁢                                              ∫                                                                              F                            ⁡                                                          (                              t                              )                                                                                ⁢                                                      exp                            ⁡                                                          (                                                              ⅈω                                ⁢                                                                  xe2x80x83                                                                ⁢                                t                                                            )                                                                                ⁢                                                      ⅆ                            t                                                                                                                +                                                                  (                                                  1                          -                          γ                                                )                                            ⁢                                              ∫                                                                              F                            ⁡                                                          (                              t                              )                                                                                ⁢                                                      exp                            ⁡                                                          (                                                              ⅈω                                ⁢                                                                  xe2x80x83                                                                ⁢                                t                                                            )                                                                                ⁢                                                      ⅆ                            t                                                                                                                                "RightBracketingBar"                                2                                                                                        =                              xe2x80x83                            ⁢                                                K                  ⁡                                      (                    f                    )                                                  ·                                                      "LeftBracketingBar"                                          ∫                                                                        F                          ⁡                                                      (                            t                            )                                                                          ⁢                                                  exp                          ⁡                                                      (                                                          ⅈω                              ⁢                                                              xe2x80x83                                                            ⁢                              t                                                        )                                                                          ⁢                                                  ⅆ                          t                                                                                      "RightBracketingBar"                                    2                                                                                        (        11        )                                          wherein          ⁢                      xe2x80x83                    ⁢          a          ⁢                      xe2x80x83                    ⁢          factor          ⁢                      xe2x80x83                    ⁢          of          ⁢                      xe2x80x83                    ⁢          proportionality          ⁢                      xe2x80x83                    ⁢                      K            ⁡                          (              f              )                                ⁢                      xe2x80x83                    ⁢          is          ⁢                      xe2x80x83                    ⁢          expressed          ⁢                      xe2x80x83                    ⁢          as          ⁢                      xe2x80x83                    ⁢          below                ,                              and            ⁢                          xe2x80x83                        ⁢            ω                    =                      2            ⁢            π            ⁢                          xe2x80x83                        ⁢                          f              .                                                          xe2x80x83                                                                                    K                ⁡                                  (                  f                  )                                            =                              xe2x80x83                            ⁢                                                "LeftBracketingBar"                                                            γexp                      ⁡                                              (                        ⅈωΔτ                        )                                                              +                                          (                                              1                        -                        γ                                            )                                                        "RightBracketingBar"                                2                                                                                        =                              xe2x80x83                            ⁢                                                "LeftBracketingBar"                                                            γ                      ⁢                                              {                                                                              cos                            ⁡                                                          (                              ωΔτ                              )                                                                                +                                                      i                            ⁢                                                          xe2x80x83                                                        ⁢                                                          sin                              ⁡                                                              (                                ωΔτ                                )                                                                                                                                    }                                                              +                                          (                                              1                        -                        γ                                            )                                                        "RightBracketingBar"                                2                                                                                        =                              xe2x80x83                            ⁢                              1                -                                  4                  ⁢                                      γ                    ⁡                                          (                                              1                        -                        γ                                            )                                                        ⁢                                                            sin                      2                                        ⁡                                          (                                              π                        ⁢                                                  xe2x80x83                                                ⁢                        f                        ⁢                                                  xe2x80x83                                                ⁢                        Δτ                                            )                                                                                                                              (        12        )            
In formula (11), parameters (optical delay difference xcex94xcfx84 and optical intensity splitting ratio xcex3) relating to a PMD state are included in only K(f), and separated from the baseband spectrum |∫F(t)exp(ixcfx89t)dt|2 in the case of no PMD. When a frequency component f=fe(Hz) is extracted by a filter or the like and an intensity thereof is detected, dependency on optical delay difference xcex94xcfx84 and the optical intensity splitting ratio xcex3 is expressed by K(fe). Moreover, from that the formula (11) is established for a general formula F(t) representing an optical waveform, the above result that the PMD state can be detected with K(fe) is established irrespective of a modulating system (NRZ or RZ) or a waveform change due to such as wavelength dispersion, nonlinear effect or the like.
FIG. 75 shows a result of an experiment showing xcex94xcfx84 dependency of 20 GHz components intensity in a 40 Gb/s NRZ system in the case of xcex3=0.5. In this intensity detecting method, an optical signal is converted into an electric signal using a photo receiver (PD) in the receiving terminal, a signal of a 20 GHz component is extracted by a 20 GHz narrow-band band-pass filter (BPF), and an intensity is detected by a power meter. As shown in FIG. 75, the intensity is the maximum at optical delay difference xcex94xcfx84=0 ps, decreases with increasing the optical delay difference xcex94xcfx84, and becomes zero at the optical delay difference xcex94xcfx84=25 ps.
Using that the fe (Hz) component intensity is the maximum when the PMD state is the best, a method of feedback-controlling the polarization-mode dispersion compensator controlling the optical delay difference xcex94xcfx84 and the optical intensity splitting ratio xcex3 inserted in the optical transmission line (transmitting terminal, optical repeater and receiving terminal) according to a PMD monitor signal is possible.
Incidentally, there are publications relating to equalization as shown in {circle around (4)} through {circle around (6)} below.
{circle around (4)} publications relating to variable dispersion (equalization) compensator:
R. I. Laming et al., xe2x80x9cA Dispersion Tunable Grating in a 10-Gb/s 100-200-km-Step IndexFibe Linkxe2x80x9d, IEEE Photon. Technol. Lett., vol.8.,pp.428-430,1996. (being able to vary a dispersion compensation quantity by changing a temperature slope in a longitudinal direction of a chirped fiber grating);
M. M. Ohm et al., xe2x80x9cTunable fiber grating dispersion using a piezoelectric stackxe2x80x9d, OFC""97 WJ3. (being able to vary a dispersion compensation quantity by changing a stress in a longitudinal direction of a chirped fiber grating);
K. Takiguchi et al., xe2x80x9cPlanar Lightwave Circuit Optical Dispersion Equalizerxe2x80x9d, IEEE Photon. Technol., Lett., vol.6,no.1,pp.86-88 (PLC variable dispersion compensator);
A. Sano et al., xe2x80x9cAutomatic dispersion equalization by monitoring extracted-clockpower level In a 40-Gbit/s, 200-km transmission linexe2x80x9d ECOC""96 TuD.3.5 (discreet variable dispersion compensator in which fibers having a positive or negative dispersion value are cascade-connected by a 1xc3x974 mechanical switch);
{circle around (5)} publications relating to automatic dispersion equalizing system:
G. Ishikawa and H. Ooi, xe2x80x9cDemonstration of automatic dispersion equalization in 40-Gbit/s OTDM transmission,xe2x80x9d ECOC""98 WdCO6. (introduced in Sep. 23, 1998);
Ooi, Akiyama and Ishikawa, xe2x80x9cExperiment on 40 Gbit/s automatic dispersion equalization using a wavelength tunable laserxe2x80x9d, EIC. Soc., 1998 (Introduced in Sep. 30, 1998);
M. Tomizawa et al., xe2x80x9cAutomatic Dispersion Equalization for Installing High-Speed Optical Transmission Systemsxe2x80x9d, J. Lightwave Technol., vol.16, no.2, pp.184-191;
{circle around (6)} publications relating to automatic PMD compensating system:
H.Ooi, Y.Akiyama, G.Ishikawa, xe2x80x9cAutomatic polarization-mode dispersion compensation in 40-Gbit/s transmissionxe2x80x9d (tentative title), submitted to OFC""99 (method of using a polarization controller(PC: Polarization Controller) and a polarization maintaining fiber (PMF: Polarization Maintaining Fiber) in a receiving terminal to control PC in a 40 Gb/s NRZ system, thereby giving a delay difference of an inverse code to a transmission line);
J. H. Winters et al., xe2x80x9coptical equalization of polarization dispersionxe2x80x9d, SPIE Vol.1787 Multigigabit Fiber Communications, 1992 00.346-357 (method of using a polarization controller in a transmitting terminal, feeding-back the transmission characteristic from a receiving terminal to control in such a direction as xcex3=0 or 1)
T. Takahashi et al., xe2x80x9cAutomatic compensation technique for timewise fluctuating polarization-mode dispersion in in-line amplifier systemsxe2x80x9d, Electron. Lett., vol.30, no.4, 1994, pp.348-349 (method of giving a delay difference of an inverse code to a transmission line by using a polarization controller (PC) and a polarization maintaining fiber (PMF) in a receiving terminal to control PC), wherein a 5 GHz component intensity in a baseband spectrum of a 10 Gb/s NRZ signal is detected and a control is performed such that the intensity becomes the maximum;
T. Ono et al., xe2x80x9cPolarization Control Method for Suppressing Polarization-mode Dispersion Influence in Optical Transmission Systemsxe2x80x9d, J. Lightwave Technol., vol.12, no.5, 1994, pp.891-898 (method of using a polarization controller, a polarization beam splitter, photo receivers for respective light paths and a variable delay element giving a delay difference between both electric signals to control the PC and the variable delay element).
In the light of the above problems, an object of the present invention is to provide a polarization-mode dispersion quantity detecting method in which polarization-mode dispersion generated in a high-speed optical signal can be easily detected and monitored, a dispersion compensation controlling method in which these detected polarization-mode dispersion and chromatic dispersion can be compensated, thereby enabling a long-distance transmission of a high-speed optical signal, and a dispersion compensation controlling apparatus for simultaneously compensating transmission optical waveform deterioration caused thereby using the polarization-mode dispersion quantity detecting method and the chromatic dispersion detecting method.
Therefore, a dispersion compensation controlling apparatus of this invention comprises a first specific frequency component detecting unit for detecting a first specific frequency component in a baseband spectrum in a transmission optical signal inputted to a receiving side over a transmission fiber as a transmission line, a first intensity detecting unit for detecting information on an intensity of the first specific frequency component detected by the first specific frequency component detecting unit, and a polarization-mode dispersion controlling unit for controlling a polarization-mode dispersion quantity of the transmission line such that the intensity of the first specific frequency component detected by the first intensity detecting unit becomes the maximum.
Accordingly, it is thereby possible to compensate polarization-mode dispersion so as to prevent deterioration of a transmission waveform of an optical signal. This advantageously contributes to long-distance transmission of a high-speed optical signal.
Further, a dispersion compensation controlling apparatus of this invention comprises a first specific frequency component detecting unit for detecting a first specific frequency component in a baseband spectrum in a transmission optical signal inputted to a receiving side over a transmission fiber as a transmission line, a first intensity detecting unit for detecting information on an intensity of the first specific frequency component detected by the first specific frequency component detecting unit, a polarization-mode dispersion controlling unit for controlling a polarization-mode dispersion quantity of the transmission line such that the intensity of the first specific frequency component detected by the first intensity detecting unit becomes the maximum, a second specific frequency component detecting unit for detecting a second specific frequency component in the baseband spectrum in the transmission optical signal, a second intensity detecting unit for detecting information on the intensity of the second specific frequency component detected by the second specific frequency component detecting unit, and a chromatic dispersion controlling unit for controlling a chromatic dispersion quantity of the transmission line such that the intensity of the second specific frequency component detected by the second specific frequency intensity detecting unit becomes the maximum.
Accordingly, it is thereby possible to compensate polarization-mode dispersion to prevent deterioration of a transmission waveform of an optical signal. It is also possible to compensate chromatic dispersion of a transmission optical signal, so as to prevent deterioration of the transmission waveform of the optical signal by effects of polarization-mode dispersion and chromatic dispersion. This more advantageously contributes to long-distance transmission of a high-speed optical signal.
Still further, a dispersion compensation controlling apparatus of this invention comprises a first specific frequency component detecting unit for detecting a first specific frequency component in a baseband spectrum in a transmission optical signal inputted to a receiving side over a transmission fiber as a transmission line, a first intensity detecting unit for detecting information on an intensity of the first specific frequency component detected by the first specific frequency component detecting unit, a polarization-mode dispersion controlling unit for controlling a polarization-mode dispersion quantity of the transmission line such that the intensity of the first specific frequency component detected by the first intensity detecting unit becomes the maximum, and a chromatic dispersion controlling unit for controlling a chromatic dispersion quantity of the transmission line such that the intensity of the first specific frequency component detected by the first intensity detecting unit becomes the maximum or the minimum.
Accordingly, it is thereby possible to compensate polarization-mode dispersion to prevent deterioration of a transmission waveform of an optical signal. It is also possible to compensate chromatic dispersion of a transmission optical signal, so as to prevent deterioration of the transmission waveform of the optical signal by effects of polarization-mode dispersion and chromatic dispersion. This more advantageously contributes to long-distance transmission of a high-speed optical signal.
A polarization-mode dispersion quantity detecting method of this invention comprises the steps of a specific frequency component detecting step of detecting a specific frequency component in a baseband spectrum in a transmission optical signal inputted over a transmission optical fiber, an intensity detecting step of detecting an intensity of the specific frequency component detected at the specific frequency component detecting step, and a dispersion quantity detecting step of detecting a polarization-mode dispersion quantity of the transmission optical signal from information on the intensity of the specific frequency component detected at said intensity detecting step by performing a predetermined functional operation.
Accordingly, it is thereby possible to easily detect polarization-mode dispersion generated in a transmission optical signal.
In addition, a dispersion compensation controlling method of this invention comprises the steps of a first specific frequency component detecting step of detecting a first specific frequency component in a baseband spectrum in a transmission optical signal inputted to a receiving side over a transmission fiber as a transmission line, a first intensity detecting step of detecting information on an intensity of the first specific frequency component detected at the first specific frequency component detecting step, and a polarization-mode dispersion controlling step of controlling a polarization-mode dispersion quantity of the transmission line such that the intensity of the first specific frequency component detected at the first intensity detecting step becomes the maximum.
Accordingly, it is thereby possible to easily detected polarization-mode dispersion generated in a transmission optical signal.
Further, a dispersion compensation controlling method comprises the steps of a first specific frequency component detecting step of detecting a first specific frequency component in a baseband spectrum in a transmission optical signal inputted to a receiving side over a transmission fiber as a transmission line, a first intensity detecting step of detecting information on an intensity of the first specific frequency component detected at the first specific frequency component detecting step, a polarization-mode dispersion controlling step of controlling a polarization-mode dispersion quantity of the transmission line such that the intensity of the first specific frequency component detected at the first intensity detecting step becomes the maximum, a second specific frequency component detecting step of detecting a second specific frequency component in the baseband spectrum in the transmission optical signal, a second intensity detecting step of detecting information on an intensity of the second specific frequency component detected at the second specific frequency component detecting step, and a chromatic dispersion controlling step of controlling a chromatic dispersion quantity of the transmission line such that the intensity of the second specific frequency component detected at the second intensity detecting step becomes the maximum or the minimum.
Accordingly, it is thereby possible to perform the controls independently and simultaneously.
Still further, a dispersion compensation controlling method of this invention comprises the steps of a first specific frequency component detecting step of detecting a first specific frequency component in a baseband spectrum in a transmission optical signal inputted to a receiving side over a transmission fiber as a transmission line, a first intensity detecting step of detecting information on an intensity of the first specific frequency component detected at the first specific frequency component detecting step, a polarization-mode dispersion controlling step of controlling a polarization-mode dispersion quantity of said transmission line such that the intensity of the first specific frequency component detected at the first intensity detecting step becomes the maximum, and a chromatic dispersion controlling step of controlling a chromatic dispersion quantity of the transmission line such that the intensity of the first specific frequency component detected at the first intensity detecting step becomes the maximum or the minimum.
Accordingly, it is thereby possible to prevent deterioration of a transmission waveform of an optical signal by effects of polarization-mode dispersion and chromatic dispersion, which further contributes to long-distance transmission of a high-speed optical signal.